Intracellular mechanics of migrating fibroblasts

Investigating longitudinal changes in the mechanical properties of MCF-7 cells exposed to paclitaxol using particle tracking microrheology

Evidence suggests that compression and shear wave elastography are sensitive to the mechanical property changes occuring in dying cells following chemotherapy, and can hence be used to monitor cancer treatment response. A qualitative and quantitative understanding of the mechanical changes at the cellular level would allow to better infer how these changes affect macroscopic tissue mechanical properties and therefore allow the optimization of elastographic techniques (such as shear wave elastography) for the monitoring of cancer therapy. We used intracellular particle tracking microrheology (PTM) to investigate the mechanical property changes of cells exposed to paclitaxol, a mitotic inhibitor used in cancer chemotherapy. The average elastic and viscous moduli of the cytoplasm of treated MCF-7 breast cancer cells were calculated for frequency ranges between 0.2 and 100 rad s(-1) (corresponding to 0.03 and 15.92 Hz, respectively). A significant increase in the complex shear modulus of the cell cytoplasm was detected at 12 h post treatment. At 24 h after drug exposure, the elastic and viscous moduli increased by a total of 191.3 Pa (>8000×) and 9 Pa (∼9×), respectively for low frequency shear modulus measurements (at 1 rad s(-1)). At higher frequencies (10 rad s(-1)), the elastic and viscous moduli increased by 188.5 Pa (∼60×) and 1.7 Pa (∼1.1×), respectively. Our work demonstrates that PTM can be used to measure changes in the mechanical properties of treated cells and that cell elasticity significantly increases by 24 h after chemotherapy exposure.

Alternatives for animal wound model systems

In this chapter a review of animal model systems already being utilized to study normal and pathologic wound healing is provided. We also go into details on alternatives for animal wound model systems. The case is made for limitations in the various approaches. We also discuss the benefits/limitations of in vitro/ex vivo systems bringing everything up to date with our current work on developing a cell-based reporter system for diabetic wound healing.

Biomechanical analysis predicts decreased human mesenchymal stem cell function before molecular differences

Multipotent human mesenchymal stem cells (hMSCs) are uniquely suited for the growing field of regenerative medicine due to their ease of isolation, expansion, and transplantation. However, during ex vivo expansion necessary to obtain clinically relevant cell quantities, hMSCs undergo fundamental changes culminating in cellular senescence. The molecular changes as hMSCs transition into senescence have been well characterized, but few studies have focused on the mechanical properties that govern many processes necessary for therapeutic efficacy. We show that before detectable differences in classical senescence markers emerge, single-cell mechanical and cytoskeletal properties reveal a subpopulation of 'non-functioning' hMSCs that appears after even limited expansion. This subpopulation, characterized by a loss of dynamic cytoskeletal stiffening and morphological flexibility in response to chemotactic signals grows with extended culture resulting in overall decreased hMSC motility and ability to contract collagen gels. These changes were mitigated with cytoskeletal inhibition. Finally, a xenographic wound healing model was used to demonstrate that these in vitro differences correlate with decreased ability of hMSCs to aid in wound closure in vivo. These data illustrate the importance of analyzing not only the molecular markers, but also mechanical markers of hMSCs as they are investigated for potential therapeutics.

Cell migration

Cell migration is fundamental to establishing and maintaining the proper organization of multicellular organisms. Morphogenesis can be viewed as a consequence, in part, of cell locomotion, from large-scale migrations of epithelial sheets during gastrulation, to the movement of individual cells during development of the nervous system. In an adult organism, cell migration is essential for proper immune response, wound repair, and tissue homeostasis, while aberrant cell migration is found in various pathologies. Indeed, as our knowledge of migration increases, we can look forward to, for example, abating the spread of highly malignant cancer cells, retarding the invasion of white cells in the inflammatory process, or enhancing the healing of wounds. This article is organized in two main sections. The first section is devoted to the single-cell migrating in isolation such as occurs when leukocytes migrate during the immune response or when fibroblasts squeeze through connective tissue. The second section is devoted to cells collectively migrating as part of multicellular clusters or sheets. This second type of migration is prevalent in development, wound healing, and in some forms of cancer metastasis.

Depth-resolved cellular microrheology using HiLo microscopy

It is increasingly important to measure cell mechanical properties in three-dimensional environments. Particle tracking microrheology (PTM) can measure cellular viscoelastic properties; however, out-of-plane data can introduce artifacts into these measurements. We developed a technique that employs HiLo microscopy to reduce out-of-plane contributions. This method eliminated signals from 90% of probes 0.5 μm or further from the focal plane, while retaining all in-plane probes. We used this technique to characterize live-cell bilayers and found that there were significant, frequency-dependent changes to the extracted cell moduli when compared to conventional analysis. Our results indicate that removal of out-of-plane information is vital for accurate assessments of cell mechanical properties.

Differential mechanical response of mesenchymal stem cells and fibroblasts to tumor-secreted soluble factors

The progression of neoplastic malignancies is a complex process resulting not only from the accumulation of mutations within tumor cells, but also modulation of the tumor microenvironment. Recent advances have shown that the recruitment and subsequent heterotypic interactions of stromal cells--including fibroblasts and bone marrow-derived mesenchymal stem cells (MSCs)--are crucial for carcinogenesis. Though extensive work has been done analyzing the signals that recruit these cells, the governing mechanical properties have not been fully investigated. Here, we report that despite their initial similarities, MSCs respond not only faster but also more dramatically to pro-migratory tumor-secreted soluble factors. Utilizing multiple particle tracking microrheology to probe the cytoskeletal mechanical properties, we show that MSCs stiffen completely within one hour, three times faster than fibroblasts. In addition, unlike fibroblasts, MSCs exposed to tumor-secreted soluble factors display a functionally different phenotype characterized by morphological elongation, decreased actin stress fiber density, and decreased adhesion. Quantitative real-time PCR indicates these phenomena occur based on differential expression of small GTPases RhoA and Cdc42, but not Rac1. These findings demonstrate a fundamental difference in the recruitment of fibroblasts and MSCs.

A new in vitro wound model based on the co-culture of human dermal microvascular endothelial cells and human dermal fibroblasts

BACKGROUND INFORMATION

Different in vitro models, based on co-culturing techniques, can be used to investigate the behaviour of cell types, which are relevant for human wound and soft-tissue healing. Currently, no model exists to describe the behaviour of fibroblasts and microvascular endothelial cells under wound-specific conditions. In order to develop a suitable in vitro model, we characterized co-cultures comprising NHDFs (normal human dermal fibroblasts) and HDMECs (human dermal microvascular endothelial cells). The CCSWMA (co-culture scratch wound migration assay) developed was supported by direct visualization techniques in order to investigate a broad spectrum of cellular parameters, such as migration and proliferation activity, the differentiation of NHDFs into MFs (myofibroblasts) and the expression of endothelin-1 and ED-A-fibronectin (extra domain A fibronectin). The cellular response to hypoxia treatment, as one of the crucial conditions in wound healing, was monitored.

RESULTS

The comparison of the HDMEC-NHDF co-culture with the respective mono-cultures revealed that HDMECs showed a lower proliferation activity when co-cultured, but their number was stable throughout a period of 48 h. NHDFs in co-culture were slightly slower at proliferating than in the mono-culture. The MF population was stable for 48 h in the co-culture, as well as in NHDF mono-culture. Co-cultures and HDMEC mono-cultures were characterized by a slower migration rate than NHDF mono-cultures. Hypoxia decreased both cell proliferation and migration in the mono-cultures, as well as in the co-cultures, indicating the general suitability of the assay. Exclusively, in co-cultures well-defined cell clusters comprising HDMECs and MFs formed at the edges of the in vitro wounds.

CONCLUSIONS

On the basis of these results, the CCSWMA developed using co-cultures, including HDMECs, NHDFs and MFs, proved to be an effective tool to directly visualize cellular interaction. Therefore, it will serve in the future to evaluate the influence of wound-healing-related factors in vitro, as shown for hypoxia in the present study.

High-throughput ballistic injection nanorheology to measure cell mechanics

High-throughput ballistic injection nanorheology is a method for the quantitative study of cell mechanics. Cell mechanics are measured by ballistic injection of submicron particles into the cytoplasm of living cells and tracking the spontaneous displacement of the particles at high spatial resolution. The trajectories of the cytoplasm-embedded particles are transformed into mean-squared displacements, which are subsequently transformed into frequency-dependent viscoelastic moduli and time-dependent creep compliance of the cytoplasm. This method allows for the study of a wide range of cellular conditions, including cells inside a 3D matrix, cell subjected to shear flows and biochemical stimuli, and cells in a live animal. Ballistic injection lasts <1 min and is followed by overnight incubation. Multiple particle tracking for one cell lasts <1 min. Forty cells can be examined in <1 h.

Magnetic nanomanipulations inside living cells compared with passive tracking of nanoprobes to get consensus for intracellular mechanics

During the last decade, the development of nanomaterials to penetrate inside living cells has been the focus of a large number of studies, with applications for the biomedical field. However, the further dynamics of these nanomaterials inside the cells is dictated by the intracellular environment and in particular its mechanical properties. The mechanical characteristics of the cell interior can be probed with either active or passive microrheological approaches. However, active intracellular microrheology is still in its infancy, owing to the difficulty of inserting probes that can be manipulated by external forces. Here we review recent active microrheology studies using magnetic nanoprobes inserted into endosomes or phagosomes as useful approaches for measuring frequency-dependent viscoelasticity, for mapping the viscoelastic landscape, as well as for identifying the contribution of individual cytoskeleton components and the influence of cell motility. The results of such direct measurements challenge the validity of more typical passive approaches in which the spontaneous displacement of embedded nanoprobes is measured. Here we discuss that one must distinguish probes suitable for use in conditions of thermal equilibrium, whose movements reflect the mechanical environment from probes that interact actively with the cytoplasm and cytoskeleton, in a state of nonequilibrium for which fluctuation-dissipation theorem no longer holds. However, when data on these probes' viscoelastic microenvironment is available, such passive probe movements can yield useful information on the forces responsible for intracellular activity.

Mechanobiology in lung epithelial cells: measurements, perturbations, and responses

Epithelial cells of the lung are located at the interface between the environment and the organism and serve many important functions including barrier protection, fluid balance, clearance of particulate, initiation of immune responses, mucus and surfactant production, and repair following injury. Because of the complex structure of the lung and its cyclic deformation during the respiratory cycle, epithelial cells are exposed to continuously varying levels of mechanical stresses. While normal lung function is maintained under these conditions, changes in mechanical stresses can have profound effects on the function of epithelial cells and therefore the function of the organ. In this review, we will describe the types of stresses and strains in the lungs, how these are transmitted, and how these may vary in human disease or animal models. Many approaches have been developed to better understand how cells sense and respond to mechanical stresses, and we will discuss these approaches and how they have been used to study lung epithelial cells in culture. Understanding how cells sense and respond to changes in mechanical stresses will contribute to our understanding of the role of lung epithelial cells during normal function and development and how their function may change in diseases such as acute lung injury, asthma, emphysema, and fibrosis.

The cytoskeleton in plasmodesmata: a role in intercellular transport?

Actin and myosin are components of the plant cell cytoskeleton that extend from cell to cell through plasmodesmata (PD), but it is unclear how they are organized within the cytoplasmic sleeve or how they might behave as regulatory elements. Early work used antibodies to locate actin and myosin to PD, at the electron microscope level, or to pitfields (aggregations of PD in the cell wall), using immunofluorescence techniques. More recently, a green fluorescent protein (GFP)-tagged plant myosin VIII was located specifically at PD-rich pitfields in cell walls. Application of actin or myosin disrupters may modify the conformation of PD and alter rates of cell-cell transport, providing evidence for a role in regulating PD permeability. Intriguingly, there is now evidence of differentiation between types of PD, some of which open in response to both actin and myosin disrupters, and others which are unaffected by actin disrupters or which close in response to myosin inhibitors. Viruses also interact with elements of the cytoskeleton for both intracellular and intercellular transport. The precise function of the cytoskeleton in PD may change during cell development, and may not be identical in all tissue types, or even in all PD within a single cell. Nevertheless, it is likely that actin- and myosin-associated proteins play a key role in regulating cell-cell transport, by interacting with cargo and loading it into PD, and may underlie the capacity for one-way transport across particular cell and tissue boundaries.

PTEN inhibition improves wound healing in lung epithelia through changes in cellular mechanics that enhance migration

The phosphoinositide-3 kinase/Akt pathway is a vital survival axis in lung epithelia. We previously reported that inhibition of phosphatase and tensin homolog deleted on chromosome 10 (PTEN), a major suppressor of this pathway, results in enhanced wound repair following injury. However, the precise cellular and biomechanical mechanisms responsible for increased wound repair during PTEN inhibition are not yet well established. Using primary human lung epithelia and a related lung epithelial cell line, we first determined whether changes in migration or proliferation account for wound closure. Strikingly, we observed that cell migration accounts for the majority of wound recovery following PTEN inhibition in conjunction with activation of the Akt and ERK signaling pathways. We then used fluorescence and atomic force microscopy to investigate how PTEN inhibition alters the cytoskeletal and mechanical properties of the epithelial cell. PTEN inhibition did not significantly alter cytoskeletal structure but did result in large spatial variations in cell stiffness and in particular a decrease in cell stiffness near the wound edge. Biomechanical changes, as well as migration rates, were mediated by both the Akt and ERK pathways. Our results indicate that PTEN inhibition rapidly alters biochemical signaling events that in turn provoke alterations in biomechanical properties that enhance cell migration. Specifically, the reduced stiffness of PTEN-inhibited cells promotes larger deformations, resulting in a more migratory phenotype. We therefore conclude that increased wound closure consequent to PTEN inhibition occurs through enhancement of cell migration that is due to specific changes in the biomechanical properties of the cell.

Inhibition of myosin II triggers morphological transition and increased nuclear motility

We investigate the effect of myosin II inhibition on cell shape and nuclear motility in cultures of mouse radial glia-like neural progenitor and rat glioma C6 cells. Instead of reducing nucleokinesis, the myosin II inhibitor blebbistatin provokes an elongated bipolar morphology and increased nuclear motility in both cell types. When myosin II is active, time-resolved traction force measurements indicate a pulling force between the leading edge and the nucleus of C6 cells. In the absence of myosin II activity, traction forces during nucleokinesis are diminished below the sensitivity threshold of our assay. By visualizing the centrosome position in C6 cells with GFP-centrin, we show that in the presence or absence of myosin II activity, the nucleus tends to overtake or lag behind the centrosome, respectively. We interpret these findings with the help of a simple viscoelastic model of the cytoskeleton consisting active contractile and passive compressed elements.

Endothelin-1 stimulates colon cancer adjacent fibroblasts

Endothelin-1 (ET-1) is produced by and stimulates colorectal cancer cells. Fibroblasts produce tumour stroma required for cancer development. We investigated whether ET-1 stimulated processes involved in tumour stroma production by colonic fibroblasts. Primary human fibroblasts, isolated from normal tissues adjacent to colon cancers, were cultured with or without ET-1 and its antagonists. Cellular proliferation, migration and contraction were measured. Expression of enzymes involved in tumour stroma development and alterations in gene transcription were determined by Western blotting and genome microarrays. ET-1 stimulated proliferation, contraction and migration (p < 0.01 v control) and the expression of matrix degrading enzymes TIMP-1 and MMP-2, but not MMP-3. ET-1 upregulated genes for profibrotic growth factors and receptors, signalling molecules, actin modulators and extracellular matrix components. ET-1 stimulated colonic fibroblast cellular processes in vitro that are involved in developing tumour stroma. Upregulated genes were consistent with these processes. By acting as a strong stimulus for tumour stroma creation, ET-1 is proposed as a target for adjuvant cancer therapy.

Crawling cells can close wounds without purse strings or signaling

When a gash or gouge is made in a confluent layer of epithelial cells, the cells move to fill in the “wound.” In some cases, such as in wounded embryonic chick wing buds, the movement of the cells is driven by cortical actin contraction (i.e., a purse string mechanism). In adult tissue, though, cells apparently crawl to close wounds. At the single cell level, this crawling is driven by the dynamics of the cell's actin cytoskeleton, which is regulated by a complex biochemical network, and cell signaling has been proposed to play a significant role in directing cells to move into the denuded area. However, wounds made in monolayers of Madin-Darby canine kidney (MDCK) cells still close even when a row of cells is deactivated at the periphery of the wound, and recent experiments show complex, highly-correlated cellular motions that extend tens of cell lengths away from the boundary. These experiments suggest a dominant role for mechanics in wound healing. Here we present a biophysical description of the collective migration of epithelial cells during wound healing based on the basic motility of single cells and cell-cell interactions. This model quantitatively captures the dynamics of wound closure and reproduces the complex cellular flows that are observed. These results suggest that wound healing is predominantly a mechanical process that is modified, but not produced, by cell-cell signaling.

Wound healing is driven by the collective migration of groups of epithelial cells. Experiments have shown that the motions of cells during wound healing are not as simple as had once been thought. Indeed, cells do not just move out to fill in the wounded area but rather undergo a number of complex but coordinated motions. Furthermore, wound healing is not just a response to chemical cues and can be driven by cells that are not immediately at the edge of the wound. In this paper, we develop a mathematical model based on the mechanical behavior of single crawling cells and also includes cell-cell adhesion. We show that this model is capable of explaining quantitatively the dynamics that occur during wound healing assays. This suggests that wound healing is largely a mechanical process where chemical signaling merely acts to augment the overall behavior.

Differences in the microrheology of human embryonic stem cells and human induced pluripotent stem cells

Embryonic and adult fibroblasts can be returned to pluripotency by the expression of reprogramming genes. Multiple lines of evidence suggest that these human induced pluripotent stem (hiPS) cells and human embryonic stem (hES) cells are behaviorally, karyotypically, and morphologically similar. Here we sought to determine whether the physical properties of hiPS cells, including their micromechanical properties, are different from those of hES cells. To this end, we use the method of particle tracking microrheology to compare the viscoelastic properties of the cytoplasm of hES cells, hiPS cells, and the terminally differentiated parental human fibroblasts from which our hiPS cells are derived. Our results indicate that although the cytoplasm of parental fibroblasts is both viscous and elastic, the cytoplasm of hiPS cells does not exhibit any measurable elasticity and is purely viscous over a wide range of timescales. The viscous phenotype of hiPS cells is recapitulated in parental cells with disassembled actin filament network. The cytoplasm of hES cells is predominantly viscous but contains subcellular regions that are also elastic. This study supports the hypothesis that intracellular elasticity correlates with the degree of cellular differentiation and reveals significant differences in the mechanical properties of hiPS cells and hES cells. Because mechanical stimuli have been shown to mediate the precise fate of differentiating stem cells, our results support the concept that stem cell "softness" is a key feature of force-mediated differentiation of stem cells and suggest there may be subtle functional differences between force-mediated differentiation of hiPS cells and hES cells.

Towards a quantitative understanding of mitotic spindle assembly and mechanics

The 'simple' view of the mitotic spindle is that it self-assembles as a result of microtubules (MTs) randomly searching for chromosomes, after which the spindle length is maintained by a balance of outward tension exerted by molecular motors on the MTs connecting centrosomes and chromosomes, and compression generated by other motors on the MTs connecting the spindle poles. This picture is being challenged now by mounting evidence indicating that spindle assembly and maintenance rely on much more complex interconnected networks of microtubules, molecular motors, chromosomes and regulatory proteins. From an engineering point of view, three design principles of this molecular machine are especially important: the spindle assembles quickly, it assembles accurately, and it is mechanically robust--yet malleable. How is this design achieved with randomly interacting and impermanent molecular parts? Here, we review recent interdisciplinary studies that have started to shed light on this question. We discuss cooperative mechanisms of spindle self-assembly, error correction and maintenance of its mechanical properties, speculate on analogy between spindle and lamellipodial dynamics, and highlight the role of quantitative approaches in understanding the mitotic spindle design.

Comparison of scanning ion conductance microscopy with atomic force microscopy for cell imaging

We present the first direct comparison of scanning ion conductance microscopy (SICM) with atomic force microscopy (AFM) for cell imaging. By imaging the same fibroblast or myoblast cell with both technologies in series, we highlight their advantages and disadvantages with respect to cell imaging. The finite imaging force applied to the sample in AFM imaging results in a coupling of mechanical sample properties into the measured sample topography. For soft samples such as cells this leads to artifacts in the measured topography and to elastic deformation, which we demonstrate by imaging whole fixed cells and cell extensions at high resolution. SICM imaging, on the other hand, has a noncontact character and can provide the true topography of soft samples at a comparable resolution.

Stress transmission within the cell

An outstanding problem in cell biology is how cells sense mechanical forces and how those forces affect cellular functions. During past decades, it has become evident that the deformable cytoskeleton (CSK), an intracellular network of various filamentous biopolymers, provides a physical basis for transducing mechanical signals into biochemical responses. To understand how mechanical forces regulate cellular functions, it is necessary to first understand how the CSK develops mechanical stresses in response to applied forces, and how those stresses are propagated through the CSK where various signaling molecules are immobilized. New experimental techniques have been developed to quantify cytoskeletal mechanics, which together with new computational approaches have given rise to new theories and models for describing mechanics of living cells. In this article, we discuss current understanding of cell biomechanics by focusing on the biophysical mechanisms that are responsible for the development and transmission of mechanical stresses in the cell and their effect on cellular functions. We compare and contrast various theories and models of cytoskeletal mechanics, emphasizing common mechanisms that those theories are built upon, while not ignoring irreconcilable differences. We highlight most recent advances in the understanding of mechanotransduction in the cytoplasm of living cells and the central role of the cytoskeletal prestress in propagating mechanical forces along the cytoskeletal filaments to activate cytoplasmic enzymes. It is anticipated that advances in cell mechanics will help developing novel therapeutics to treat pulmonary diseases like asthma, pulmonary fibrosis, and chronic obstructive pulmonary disease.

Cytoskeletal cross-linking and bundling in motor-independent contraction

Eukaryotic and prokaryotic cells use cytoskeletal proteins to regulate and modify cell shape. During cytokinesis or eukaryotic cell crawling, contractile forces are generated inside the cell to constrict the division site or to haul the rear of the cell forward, respectively. In many cases, these forces have been attributed to the activity of molecular motors, such as myosin II, which, by pulling on actin filaments, can produce contraction of the actin cytoskeleton. However, prokaryotic division is driven by the tubulin-like protein FtsZ and does not seem to require additional molecular motors to constrict the division site. Likewise, Dictyostelium discoideum and Saccharomyces cerevisiae can perform cytokinesis under motor-free conditions. In addition, many crawling cells can translocate when myosin is inhibited or absent. In this review, we point out another force-generation mechanism that can play a significant role in driving these processes in eukaryotes and prokaryotes. This mechanism is mediated by cross-linking and bundling proteins that form effective interactions between cytoskeletal filaments. Some recent studies in this area are reviewed and the physical underpinnings of this force-generation mechanism are explained.

Analysis of video-based microscopic particle trajectories using Kalman filtering

The fidelity of the trajectories obtained from video-based particle tracking determines the success of a variety of biophysical techniques, including in situ single cell particle tracking and in vitro motility assays. However, the image acquisition process is complicated by system noise, which causes positioning error in the trajectories derived from image analysis. Here, we explore the possibility of reducing the positioning error by the application of a Kalman filter, a powerful algorithm to estimate the state of a linear dynamic system from noisy measurements. We show that the optimal Kalman filter parameters can be determined in an appropriate experimental setting, and that the Kalman filter can markedly reduce the positioning error while retaining the intrinsic fluctuations of the dynamic process. We believe the Kalman filter can potentially serve as a powerful tool to infer a trajectory of ultra-high fidelity from noisy images, revealing the details of dynamic cellular processes.

Triggering cell detachment from patterned electrode arrays by programmed subcellular release

Programmed subcellular release is an in vitro technique for the quantitative study of cell detachment. The dynamics of cell contraction are measured by releasing cells from surfaces to which they are attached with spatial and temporal control. Release of subcellular regions of cells is achieved by plating cells on an electrode array created by standard microfabrication methods. The electrodes are then biochemically functionalized with an arginine-glycine-aspartic acid (RGD)-terminated thiol. Application of a voltage pulse results in electrochemical desorption of the RGD-terminated thiols, triggering cell detachment. This method allows for the study of the full cascade of events from detachment to subsequent subcellular reorganization. Fabrication of the electrode arrays may take 1-2 d. Preparation for experiments, including surface functionalization and cell plating, can be completed in 10 h. A series of cell release experiments on one device may last several hours.

A distinctive role for focal adhesion proteins in three-dimensional cell motility

Focal adhesions are large multi-protein assemblies that form at the basal surface of cells on planar dishes, and that mediate cell signalling, force transduction and adhesion to the substratum. Although much is known about focal adhesion components in two-dimensional (2D) systems, their role in migrating cells in a more physiological three-dimensional (3D) matrix is largely unknown. Live-cell microscopy shows that for cells fully embedded in a 3D matrix, focal adhesion proteins, including vinculin, paxillin, talin, alpha-actinin, zyxin, VASP, FAK and p130Cas, do not form aggregates but are diffusely distributed throughout the cytoplasm. Despite the absence of detectable focal adhesions, focal adhesion proteins still modulate cell motility, but in a manner distinct from cells on planar substrates. Rather, focal adhesion proteins in matrix-embedded cells regulate cell speed and persistence by affecting protrusion activity and matrix deformation, two processes that have no direct role in controlling 2D cell speed. This study shows that membrane protrusions constitute a critical motility/matrix-traction module that drives cell motility in a 3D matrix.

Epithelial repair mechanisms in the lung

The recovery of an intact epithelium following lung injury is critical for restoration of lung homeostasis. The initial processes following injury include an acute inflammatory response, recruitment of immune cells, and epithelial cell spreading and migration upon an autologously secreted provisional matrix. Injury causes the release of factors that contribute to repair mechanisms including members of the epidermal growth factor and fibroblast growth factor families (TGF-alpha, KGF, HGF), chemokines (MCP-1), interleukins (IL-1beta, IL-2, IL-4, IL-13), and prostaglandins (PGE(2)), for example. These factors coordinate processes involving integrins, matrix materials (fibronectin, collagen, laminin), matrix metalloproteinases (MMP-1, MMP-7, MMP-9), focal adhesions, and cytoskeletal structures to promote cell spreading and migration. Several key signaling pathways are important in regulating these processes, including sonic hedgehog, Rho GTPases, MAP kinase pathways, STAT3, and Wnt. Changes in mechanical forces may also affect these pathways. Both localized and distal progenitor stem cells are recruited into the injured area, and proliferation and phenotypic differentiation of these cells leads to recovery of epithelial function. Persistent injury may contribute to the pathology of diseases such as asthma, chronic obstructive pulmonary disease, and pulmonary fibrosis. For example, dysregulated repair processes involving TGF-beta and epithelial-mesenchymal transition may lead to fibrosis. This review focuses on the processes of epithelial restitution, the localization and role of epithelial progenitor stem cells, the initiating factors involved in repair, and the signaling pathways involved in these processes.

Multiple-Particle-Tracking to investigate viscoelastic properties in living cells

Cell mechanical properties play an important role in determining many cellular activities. Passive microrheology techniques, such as Multiple-Particle-Tracking (MPT) give an insight into the structural rearrangements and viscoelastic response of a wide range of materials, in particular soft materials and complex fluids like cell cytoplasm in living cells. The technique finds an important field of application in large cells such as oocytes where, during their growth, several organelles and molecules are displaced in specific territories of the cell instrumental for later embryonic development. To measure cell mechanics, cells are usually deformed by many techniques that are slow and often invasive. To overcome these limits, the MPT technique is applied. Probe particles are embedded in the viscoelastic sample and their properties are extracted from the thermal fluctuation spectra measured using digital video-microscopy. The Brownian motion of a probe particle immersed in a network is directly related to the network's mechanical properties. Particles exhibit larger motions when their local environments are less rigid or less viscous. The mean-square-displacement (MSD) of the particle's trajectory is used to quantify its amplitude of motions over different time scales.

Actin-myosin viscoelastic flow in the keratocyte lamellipod

The lamellipod, the locomotory region of migratory cells, is shaped by the balance of protrusion and contraction. The latter is the result of myosin-generated centripetal flow of the viscoelastic actin network. Recently, quantitative flow data was obtained, yet there is no detailed theory explaining the flow in a realistic geometry. We introduce models of viscoelastic actin mechanics and myosin transport and solve the model equations numerically for the flat, fan-shaped lamellipodial domain of keratocytes. The solutions demonstrate that in the rapidly crawling cell, myosin concentrates at the rear boundary and pulls the actin network inward, so the centripetal actin flow is very slow at the front, and faster at the rear and at the sides. The computed flow and respective traction forces compare well with the experimental data. We also calculate the graded protrusion at the cell boundary necessary to maintain the cell shape and make a number of other testable predictions. We discuss model implications for the cell shape, speed, and bi-stability.

Intracellular particle tracking as a tool for tumor cell characterization

We studied the dynamics of two types of intracellular probe particles, ballistically injected latex spheres and endogenous granules, in tumor cell lines of different metastatic potential: breast tumor cells (MCF-7 malignant, MCF-10A benign) and pancreas adenocarcinoma (PaTu8988T malignant, PaTu8988S benign). For both tissue types and for both probes, the mean squared displacement (MSD) function measured in the malignant cells was substantially larger than in the benign cells. Only a few cells were needed to characterize the tissue as malignant or benign based on their MSD, since variations in MSD within the same cell line were relatively small. These findings suggest that intracellular particle tracking (IPT) can serve as a simple and reliable method for characterization of cell states obtained from a small amount of cell sample. Mechanical analysis of the same cell lines with atomic force microscopy (AFM) in force-distance mode revealed that AFM could distinguish between the benign and malignant breast cancer cells but not the pancreatic tumor cell lines. This underlines the potential value of IPT as a complementary nanomechanical tool for studying cell-state-dependent mechanical properties.

Nanoscale topography-induced modulation of fundamental cell behaviors of rabbit corneal keratocytes, fibroblasts, and myofibroblasts

PURPOSE

Keratocyte-to-myofibroblast differentiation is a key factor in corneal wound healing. The purpose of this study was to determine the influence of environmental nanoscale topography on keratocyte, fibroblast, and myofibroblast cell behavior.

METHODS

Primary rabbit corneal keratocytes, fibroblasts, and myofibroblasts were seeded onto planar polyurethane surfaces with six patterned areas, composed of anisotropically ordered grooves and ridges with a 400-, 800-, 1200-, 1600-, 2000-, and 4000-nm pitch (pitch = groove + ridge width). After 24 hours cells were fixed, stained, imaged, and analyzed for cell shape and orientation. For migration studies, cells on each patterned surface were imaged every 10 minutes for 12 hours, and individual cell trajectories and migration rates were calculated.

RESULTS

Keratocytes, fibroblasts, and myofibroblasts aligned and elongated to pitch sizes larger than 1000 nm. A lower limit to the topographic feature sizes that the cells responded to was identified for all three phenotypes, with a transition zone around the 800- to 1200-nm pitch size. Fibroblasts and myofibroblasts migrated parallel to surface ridges larger than 1000 nm but lacked directional guidance on submicron and nanoscale topographic features and on planar surfaces. Keratocytes remained essentially immobile.

CONCLUSIONS

Corneal stromal cells elongated, aligned, and migrated, differentially guided by substratum topographic features. All cell types failed to respond to topographic features approximating the dimensions of individual stromal fibers. These findings contribute to our understanding of corneal stromal cell biology in health and disease and their interaction with biomaterials and their native extracellular matrix.

Global and local regulation of clathrin-coated pit dynamics detected on patterned substrates

Live-cell imaging of individual clathrin-coated pit (CCP) dynamics has revealed a broad variation in their internalization kinetics, but the functional significance and mechanistic underpinnings of this heterogeneity remain unknown. One contributing factor may be the spatial variations in the underlying actin cortex. To test this, we cultured cells on fibronectin (Fn) micropatterned substrates to vary the cortical actin mechanics in a defined manner. Under these conditions, stress fibers became organized to bridge adhesive islands, creating spatial heterogeneity in the cortical actin architecture. CCP lifetimes within the Fn-coated islands were selectively prolonged. This differential effect was not due to adherence to Fn-coated surfaces, and was not observed in cells grown on patterned surfaces that did not induce organized stress fiber assembly. Pharmacological agents that lower cortical tension selectively lowered CCP lifetimes within Fn islands, thus abolishing the spatial heterogeneity in the CCP dynamics. Although we cannot rule out the possibility that other factors might locally affect CCP dynamics at Fn islands, our data suggest that localized modulation in cortical tension may spatially regulate clathrin-mediated endocytosis.

Cell mechanics: dissecting the physical responses of cells to force

It is now widely appreciated that normal tissue morphology and function rely upon cells' ability to sense and generate forces appropriate to their correct tissue context. Although the effects of forces on cells have been studied for decades, our understanding of how those forces propagate through and act on different cell substructures remains at an early stage. The past decade has seen a resurgence of interest, with a variety of different micromechanical methods in current use that probe cells' dynamic deformation in response to a time-varying force. The ability of researchers to carefully measure the mechanical properties of cells subjected to a variety of pharmacological and genetic interventions, however, currently outstrips our ability to quantitatively interpret the data in many cases. Despite these challenges, the stage is now set for the development of detailed models for cell deformability, motility, and mechanosensing that are rooted at the molecular level.

Resolving the role of actoymyosin contractility in cell microrheology

Einstein's original description of Brownian motion established a direct relationship between thermally-excited random forces and the transport properties of a submicron particle in a viscous liquid. Recent work based on reconstituted actin filament networks suggests that nonthermal forces driven by the motor protein myosin II can induce large non-equilibrium fluctuations that dominate the motion of particles in cytoskeletal networks. Here, using high-resolution particle tracking, we find that thermal forces, not myosin-induced fluctuating forces, drive the motion of submicron particles embedded in the cytoskeleton of living cells. These results resolve the roles of myosin II and contractile actomyosin structures in the motion of nanoparticles lodged in the cytoplasm, reveal the biphasic mechanical architecture of adherent cells—stiff contractile stress fibers interdigitating in a network at the cell cortex and a soft actin meshwork in the body of the cell, validate the method of particle tracking-microrheology, and reconcile seemingly disparate atomic force microscopy (AFM) and particle-tracking microrheology measurements of living cells.

The regulatory role of cell mechanics for migration of differentiating myeloid cells

Migration of cells is important for tissue maintenance, immune response, and often altered in disease. While biochemical aspects, including cell adhesion, have been studied in detail, much less is known about the role of the mechanical properties of cells. Previous measurement methods rely on contact with artificial surfaces, which can convolute the results. Here, we used a non-contact, microfluidic optical stretcher to study cell mechanics, isolated from other parameters, in the context of tissue infiltration by acute promyelocytic leukemia (APL) cells, which occurs during differentiation therapy with retinoic acid. Compliance measurements of APL cells reveal a significant softening during differentiation, with the mechanical properties of differentiated cells resembling those of normal neutrophils. To interfere with the migratory ability acquired with the softening, differentiated APL cells were exposed to paclitaxel, which stabilizes microtubules. This treatment does not alter compliance but reduces cell relaxation after cessation of mechanical stress six-fold, congruent with a significant reduction of motility. Our observations imply that the dynamical remodeling of cell shape required for tissue infiltration can be frustrated by stiffening the microtubular system. This link between the cytoskeleton, cell mechanics, and motility suggests treatment options for pathologies relying on migration of cells, notably cancer metastasis.

A novel approach to high accuracy of video-based microrheology

Video-based particle tracking monitors the microscopic movement of labeled biomolecules and fluorescent probes within a complex cellular environment. Information gained from this technique enables us to extract the dynamic behavior of biomolecules and the local mechanical properties inside the cell from a tracked particle's mean-square displacement (MSD). However, MSD measurements are highly susceptible to static error introduced by noise in the image acquisition process that leads to an incorrect positioning of the particle. Static error can mask the subtle effects from the local microenvironment on the MSD and potentially generate misleading conclusions about the biophysical properties of cells. An approach that greatly increases the accuracy of MSD measurements is presented herein by combining experimental data with Monte Carlo simulations to eliminate the inherent static error. This practical method of static error correction greatly advances particle-tracking techniques.

Characterization of cell mechanical properties by computational modeling of parallel plate compression

A substantial body of work has been reported in which the mechanical properties of adherent cells were characterized using compression testing in tandem with computational modeling. However, a number of important issues remain to be addressed. In the current study, using computational analyses, the effect of cell compressibility on the force required to deform spread cells is investigated and the possibility that stiffening of the cell cytoplasm occurs during spreading is examined based on published experimental compression test data. The effect of viscoelasticity on cell compression is considered and difficulties in performing a complete characterization of the viscoelastic properties of a cell nucleus and cytoplasm by this method are highlighted. Finally, a non-linear force-deformation response is simulated using differing linear viscoelastic properties for the cell nucleus and the cell cytoplasm.

Particle-tracking microrheology of living cells: principles and applications

A multitude of cellular and subcellular processes depend critically on the mechanical deformability of the cytoplasm. We have recently introduced the method of particle-tracking microrheology, which measures the viscoelastic properties of the cytoplasm locally and with high spatiotemporal resolution. Here we establish the basic principles of particle-tracking microrheology, describing the advantages of this approach over more conventional approaches to cell mechanics. We present basic concepts of molecular mechanics and polymer physics relevant to the microrheological response of cells. Particle-tracking microrheology can probe the mechanical properties of live cells in experimentally difficult, yet more physiological, environments, including cells embedded inside a 3D matrix, adherent cells subjected to shear flows, and cells inside a developing embryo. Particle-tracking microrheology can readily reveal the lost ability of diseased cells to resist shear forces.

Effects of green tea extract on lung cancer A549 cells: proteomic identification of proteins associated with cell migration

Green tea polyphenols exhibit multiple antitumor activities, and the mechanisms of action are not completely understood. Previously, we reported that green tea extract (GTE)-induced actin remolding is associated with increased cell adhesion and decreased motility in A549 lung cancer cells. To identify the cellular targets responsible for green tea-induced actin remodeling, we performed 2-DE LC-MS/MS of A549 cells before and after GTE exposure. We have identified 14 protein spots that changed in expression (> or =2-fold) after GTE treatment. These proteins are involved in calcium-binding, cytoskeleton and motility, metabolism, detoxification, or gene regulation. In particular we found upregulation of several genes that modulate actin remodeling and cell migration, including lamin A/C. Our data indicated that GTE-induced lamin A/C upregulation appears to be at the transcriptional level and the increased expression results in the decrease in cell motility, as confirmed by siRNA. The result of the study demonstrates that GTE alters the levels of many proteins involved in growth, motility and apoptosis of A549 cells and their identification may explain the multiple antitumor activities of GTE.

Mapping of spatiotemporal heterogeneous particle dynamics in living cells

Colloidal particles embedded in the cytoplasm of living mammalian cells have been found to display remarkable heterogeneity in their amplitude of motion. However, consensus on the significance and origin of this phenomenon is still lacking. We conducted experiments on Hmec-1 cells loaded with about 100 particles to reveal the intracellular particle dynamics as a function of both location and time. Central quantity in our analysis is the amplitude (A) of the individual mean-squared displacement (iMSD), averaged over a short time. Histograms of A were measured, (1) over all particles present at the same time and (2) for individual particles as a function of time. Both distributions showed significant broadening compared to particles in Newtonian liquid, indicating that the particle dynamics varies with both location and time. However, no systematic dependence of A on intracellular location was found. Both the (strong) spatial and (weak) temporal variations were further analyzed by correlation functions of A . Spatial cross correlations were rather weak down to interparticle distances of 1 microm , suggesting that the precise intracellular probe distribution is not crucial for observing a dynamic behavior that is representative for the whole cell. Temporal correlations of A decayed at approximately 10 s , possibly suggesting an intracellular reorganization at this time scale. These findings imply (1) that both individual particle dynamics and the ensemble averaged behavior in a given cell can be measured if there are enough particles per cell and (2) that the amplitude and power-law exponent of iMSDs can be used to reveal local dynamics. We illustrate this by showing how superdiffusive and subdiffusive behaviors may be hidden under an apparently diffusive global dynamics.

Biological studies on Brazilian plants used in wound healing

AIM OF THE STUDY

n-Hexanic and ethanolic extracts from twelve plants (Brugmansia suaveolens Brecht. et Presl., Eupatorium laevigatum Lam., Galinsoga parviflora Cav., Iresine herbstii Hook., Kalanchöe tubiflora Hamet-Ahti, Petiveria alliacea L., Pluchea sagittalis (Lam.) Cabrera, Piper regnellii DC., Schinus molle L., Sedum dendroideum Moç et Sessé ex DC., Waltheria douradinha St. Hill., Xanthium cavanillesii Schouw.) used in traditional South Brazilian medicine as wound healing agents were investigated in various biological assays, targeting different aspects in this complex process.

MATERIALS AND METHODS

The extracts were investigated on NF-kappaB DNA binding, p38alpha MAPK, TNF-alpha release, direct elastase inhibition and its release as well as on caspase-3. Fibroblasts migration to and proliferation into the wounded monolayers were evaluated in the scratch assay, the agar diffusion test for antibacterial and the MTT assay for cytotoxic effects.

RESULTS

The hydrophilic extracts from Galinsoga parviflora, Petiveria alliacea, Schinus molle, Waltheria douradinha and Xanthium cavanillesii as well as the lipophilic extract of Waltheria douradinha turned out to be the most active ones.

CONCLUSIONS

These results increase our knowledge on the wound healing effects of the investigated medicinal plants. Further studies are necessary to find out the effective secondary metabolites responsible for the observed effects.

Micro- and macrorheology of mucus

Mucus is a complex biological material that lubricates and protects the human lungs, gastrointestinal (GI) tract, vagina, eyes, and other moist mucosal surfaces. Mucus serves as a physical barrier against foreign particles, including toxins, pathogens, and environmental ultrafine particles, while allowing rapid passage of selected gases, ions, nutrients, and many proteins. Its selective barrier properties are precisely regulated at the biochemical level across vastly different length scales. At the macroscale, mucus behaves as a non-Newtonian gel, distinguished from classical solids and liquids by its response to shear rate and shear stress, while, at the nanoscale, it behaves as a low viscosity fluid. Advances in the rheological characterization of mucus from the macroscopic to nanoscopic levels have contributed critical understanding to mucus physiology, disease pathology, and the development of drug delivery systems designed for use at mucosal surfaces. This article reviews the biochemistry that governs mucus rheology, the macro- and microrheology of human and laboratory animal mucus, rheological techniques applied to mucus, and the importance of an improved understanding of the physical properties of mucus to advancing the field of drug and gene delivery.

Anisotropic rheology and directional mechanotransduction in vascular endothelial cells

Adherent cells remodel their cytoskeleton, including its directionality, in response to directional mechanical stimuli with consequent redistribution of intracellular forces and modulation of cell function. We analyzed the temporal and spatial changes in magnitude and directionality of the cytoplasmic creep compliance (Gamma) in confluent cultures of bovine aortic endothelial cells subjected to continuous laminar flow shear stresses. We extended particle tracking microrheology to determine at each point in the cytoplasm the principal directions along which Gamma is maximal and minimal. Under static condition, the cells have polygonal shapes without specific alignment. Although Gamma of each cell exhibits directionality with varying principal directions, Gamma averaged over the whole cell population is isotropic. After continuous laminar flow shear stresses, all cells gradually elongate and the directions of maximal and minimal Gamma become, respectively, parallel and perpendicular to flow direction. This mechanical alignment is accompanied by a transition of the cytoplasm to be more fluid-like along the flow direction and more solid-like along the perpendicular direction; at the same time Gamma increases at the downstream part of the cells. The resulting directional anisotropy and spatial inhomogeneity of cytoplasmic rheology may play an important role in mechanotransduction in adherent cells by providing a means to sense the direction of mechanical stimuli.

Dysfunctional connections between the nucleus and the actin and microtubule networks in laminopathic models

Laminopathies encompass a wide array of human diseases associated to scattered mutations along LMNA, a single gene encoding A-type lamins. How such genetic alterations translate to cellular defects and generate such diverse disease phenotypes remains enigmatic. Recent work has identified nuclear envelope proteins--emerin and the linker of the nucleoskeleton and cytoskeleton (LINC) complex--which connect the nuclear lamina to the cytoskeleton. Here we quantitatively examine the composition of the nuclear envelope, as well as the architecture and functions of the cytoskeleton in cells derived from two laminopathic mouse models, including Hutchinson-Gilford progeria syndrome (Lmna(L530P/L530P)) and Emery-Dreifuss muscular dystrophy (Lmna(-/-)). Cells derived from the overtly aphenotypical model of X-linked Emery-Dreifuss muscular dystrophy (Emd(-/y)) were also included. We find that the centrosome is detached from the nucleus, preventing centrosome polarization in cells under flow--defects that are mediated by the loss of emerin from the nuclear envelope. Moreover, while basal actin and focal adhesion structure are mildly affected, RhoA activation, cell-substratum adhesion, and cytoplasmic elasticity are greatly lowered, exclusively in laminopathic models in which the LINC complex is disrupted. These results indicate a new function for emerin in cell polarization and suggest that laminopathies are not directly associated with cells' inability to polarize, but rather with cytoplasmic softening and weakened adhesion mediated by the disruption of the LINC complex across the nuclear envelope.

Mapping local matrix remodeling induced by a migrating tumor cell using three-dimensional multiple-particle tracking

Mesenchymal cell migration through a three-dimensional (3D) matrix typically involves major matrix remodeling. The direction of matrix deformation occurs locally in all three dimensions, which cannot be measured by current techniques. To probe the local, 3D, real-time deformation of a collagen matrix during tumor cell migration, we developed an assay whereby matrix-embedded beads are tracked simultaneously in all three directions with high resolution. To establish a proof of principle, we investigated patterns of collagen I matrix deformation near fibrosarcoma cells in the absence and presence of inhibitors of matrix metalloproteinases and acto-myosin contractility. Our results indicate that migrating cells show patterns of local matrix deformation toward the cell that are symmetric in magnitude with respect to the axis of cell movement. In contrast, patterns of matrix release from the cell are asymmetric: the matrix is typically relaxed first at the back of the cell, allowing forward motion, and then at the cell's leading edge. Matrix deformation in regions of the matrix near the cell's leading edge is elastic and mostly reversible, but induces irreversible matrix rupture events near the trailing edge. Our results also indicate that matrix remodeling spatially correlates with protrusive activity. This correlation is mediated by myosin II and Rac1, and eliminated after inhibition of pericellular proteolysis or ROCK. We have developed an assay based on high-resolution 3D multiple-particle tracking that allows us to probe local matrix remodeling during mesenchymal cell migration through a 3D matrix and simultaneously monitor protrusion dynamics.

Localized elasticity measured in epithelial cells migrating at a wound edge using atomic force microscopy

Restoration of lung homeostasis following injury requires efficient wound healing by the epithelium. The mechanisms of lung epithelial wound healing include cell spreading and migration into the wounded area and later cell proliferation. We hypothesized that mechanical properties of cells vary near the wound edge, and this may provide cues to direct cell migration. To investigate this hypothesis, we measured variations in the stiffness of migrating human bronchial epithelial cells (16HBE cells) approximately 2 h after applying a scratch wound. We used atomic force microscopy (AFM) in contact mode to measure the cell stiffness in 1.5-microm square regions at different locations relative to the wound edge. In regions far from the wound edge (>2.75 mm), there was substantial variation in the elastic modulus in specific cellular regions, but the median values measured from multiple fields were consistently lower than 5 kPa. At the wound edge, cell stiffness was significantly lower within the first 5 microm but increased significantly between 10 and 15 microm before decreasing again below the median values away from the wound edge. When cells were infected with an adenovirus expressing a dominant negative form of RhoA, cell stiffness was significantly decreased compared with cells infected with a control adenovirus. In addition, expression of dominant negative RhoA abrogated the peak increase in stiffness near the wound edge. These results suggest that cells near the wound edge undergo localized changes in cellular stiffness that may provide signals for cell spreading and migration.

Intracellular microrheology of motile Amoeba proteus

The motility of Amoeba proteus was examined using the technique of passive particle tracking microrheology, with the aid of newly developed particle tracking software, a fast digital camera, and an optical microscope. We tracked large numbers of endogeneous particles in the amoebae, which displayed subdiffusive motion at short timescales, corresponding to thermal motion in a viscoelastic medium, and superdiffusive motion at long timescales due to the convection of the cytoplasm. Subdiffusive motion was characterized by a rheological scaling exponent of 3/4 in the cortex, indicative of the semiflexible dynamics of the actin fibers. We observed shear-thinning in the flowing endoplasm, where exponents increased with increasing flow rate; i.e., the endoplasm became more fluid-like. The rheology of the cortex is found to be isotropic, reflecting an isotropic actin gel. A clear difference was seen between cortical and endoplasmic layers in terms of both viscoelasticity and flow velocity, where the profile of the latter is close to a Poiseuille flow for a Newtonian fluid.

Round versus flat: bone cell morphology, elasticity, and mechanosensing

There is increasing evidence that cell function and mechanical properties are closely related to morphology. However, most in vitro studies investigate flat adherent cells, which might not reflect physiological geometries in vivo. Osteocytes, the mechanosensors in bone, reside within ellipsoid containment, while osteoblasts adhere to flatter bone surfaces. It is unknown whether morphology difference, dictated by the geometry of attachment is important for cell rheology and mechanosensing. We developed a novel methodology for investigating the rheology and mechanosensitivity of bone cells under different morphologies using atomic force microscopy and our two-particle assay for optical tweezers. We found that the elastic constant of MLO-Y4 osteocytes when flat and adherent (>1 kPa) largely differed when round but partially adherent (<1 kPa). The elastic constant of round suspended MLO-Y4 osteocytes, MC3T3-E1 osteoblasts, and primary osteoblasts were similarly <1 kPa. The mechanosensitivity of round suspended MLO-Y4 osteocytes was investigated by monitoring nitric oxide (NO) release, an essential signaling molecule in bone. A preliminary observation of high NO release from round suspended MLO-Y4 osteocytes in response to approximately 5 pN force is reported here, in contrast with previous studies where flat cells routinely release lesser NO while being stimulated with higher force. Our results suggest that a round cellular morphology supports a less stiff cytoskeleton configuration compared with flat cellular morphology. This implies that osteocytes take advantage of their ellipsoid morphology in vivo to sense small strains benefiting bone health. Our assay provides novel opportunities for in vitro studies under a controlled suspended morphology versus commonly studied adherent morphologies.

Cell stiffening in response to external stress is correlated to actin recruitment

We designed a micromanipulation device that allows the local application of a constant force on living cells, and the measurement of their stiffness. The force is applied through an Arg-Gly-Asp-coated bead adhering on the cell and trapped in optical tweezers controlled by a feedback loop. Epifluorescence observations of green fluorescent protein-actin in the cells are made during force application. We observe a stiffening of cells submitted to a constant force within a few minutes, coupled to actin recruitment both at the bead-cell contact and up to several micrometers from the stress application zone. Moreover, kinetics of stiffening and actin recruitment exhibit a strong correlation. This work presents the first quantification of the dynamics of cell mechanical reinforcement under stress, which is a novel insight into the elucidation of the more general phenomenon of cell adaptation to stress.

Brushes, cables, and anchors: recent insights into multiscale assembly and mechanics of cellular structural networks

The remarkable ability of living cells to sense, process, and respond to mechanical stimuli in their environment depends on the rapid and efficient interconversion of mechanical and chemical energy at specific times and places within the cell. For example, application of force to cells leads to conformational changes in specific mechanosensitive molecules which then trigger cellular signaling cascades that may alter cellular structure, mechanics, and migration and profoundly influence gene expression. Similarly, the sensitivity of cells to mechanical stresses is governed by the composition, architecture, and mechanics of the cellular cytoskeleton and extracellular matrix (ECM), which are in turn driven by molecular-scale forces between the constituent biopolymers. Understanding how these mechanochemical systems coordinate over multiple length and time scales to produce orchestrated cell behaviors represents a fundamental challenge in cell biology. Here, we review recent advances in our understanding of these complex processes in three experimental systems: the assembly of axonal neurofilaments, generation of tensile forces by actomyosin stress fiber bundles, and mechanical control of adhesion assembly.